1. Field of the Invention
The present invention relates to an apparatus for interfacing between different networks.
2. Description of the Related Art
Currently there are networks using asynchronous signals (hereinafter called a DSn network) and networks using synchronous (SONET) signals (hereinafter called a SONET network) in the transmission networks of North America, etc. When a DSn network and a SONET network are connected, a SONET optical transmission apparatus is used. Although an apparatus for connecting a network using-a DS3 (44.736 Mbps) signal (hereinafter called a DS3 network) to a network in which signals can be demultiplexed and multiplexed in units of STS1 (51.84 Mbps, a frame format of a SONET network) in the SONET network (hereinafter called a SONET STS1 network), is implemented, an apparatus for connecting a network using the DS3 (44.736 Mbps) signal to a network in which signals can be demultiplexed and multiplexed in units of VT1.5 (1.728 Mbps, a signal provided to easily add/drop-multiplex the payload of an STS signal, given the abbreviation VT of virtual tributary) in the SONET network (hereinafter called a SONET VT network), is not implemented yet.
That is, a conventional apparatus can multiplex and demultiplex DS3 signals received from a DSn network only in units of STS1 units (51.84 Mbps) in the SONET network, but cannot multiplex and demultiplex the DS3 signals in units of VT1.5 units (1.728 Mbps).
The SONET network and the DS3 network are formed independently of each other. Therefore, if a DS3 signal is connected to the SONET network, the conventional apparatus cannot monitor the condition of the DS2 signal in a DS3 signal, nor a DS1 signal in the DS3 network, though the apparatus can monitor the condition of the connecting signal of the DS3 signal.
Recently there has been an increasing demand for an apparatus in which signals can be multiplexed and demultiplexed in units of VT1.5 while operating the DS3 network, and that which can enter a SONET VT network. It is an object of the present invention to provide an apparatus for converting DS3 signals to VT1.5 signals, and constructing a VT1.5 network so that an apparatus supporting the SONET VT network can receive the services of a network using the DS3 (44.736 Mbps) signal.
FIG. 1 shows the configuration in the case where a SONET VT network is connected to a DS3 network by combining a conventional M13 apparatus and a conventional ADM apparatus. First, an STS1 input signal from the SONET VT network is demultiplexed and converted by an ADM 2700. The STS1 signal transmitted from a SONET network is first demultiplexed and converted to VT signals by an STS1/VT1.5 demultiplexer unit 2702, and further demultiplexed and converted to DS1 signals by a VT1.5/DS1 demultiplexer unit 2703. When the VT signals are demultiplexed and converted to the DS1 signals, the signals stored in the memory of the VT1.5/DS1 demultiplexer unit 2703 are output synchronized with a clock generated by a clock generator unit for the DS1 network 2704. When the DS1 signals demultiplexed and converted in this way are multiplexed and converted to a number of DS2 signals, the DS1 signals are stored once in a memory of an M13 apparatus, read using a DS2 clock, and multiplexed to the DS2 signals. Furthermore, these DS2 signals are synchronized with a clock signal from a clock generator unit for the DS3 network 2707, are stored once in the memory of a DS2/DS3 multiplexer unit 2706, are read using a clock for a DS3 network, are multiplexed and converted to a DS3 signal, and sent out to the DS3 network. However, the provision of a memory for 28 channels of DS1 in an apparatus in order to multiplex and convert DS1 signals to DS2 signals leads to large-scale circuitry and an increased consumption of power.
FIG. 2 shows the configuration in the case where a network using a DS3 (44.736 Mbps) signal is connected to a SONET network by using a conventional apparatus. In this case, the DS3 network can be connected only to a SONET STS1 (51.84 Mbps) network, and only the condition of a DS3 signal can be monitored for an asynchronous network, and both DS2 and DS1 signals in the DS3 signal cannot be monitored.
That is, when a conventional ADM 2800 is used, the DS3 signal received from the DS3 network is converted by a DS3/STS1 multiplexer unit 2803, and the alarm signal of the DS3 signal is detected by a DS3 alarm detector unit 2802, which is reported to a SONET condition reporting unit 2801. Since the SONET condition reporting unit 2801 transmits this alarm signal to the SONET network via an intra-apparatus condition monitor unit, the alarm information of the DS3 signal is sent to the SONET network, but the alarm information of both DS2 and DS1 signals cannot be extracted.
FIG. 3 shows the configuration in the case where a DS3 network is connected to a SONET VT network by combining a conventional M13 apparatus and a conventional ADM apparatus. In this case, the condition of DS3, DS2 and DS1 signals can be monitored by the M13 apparatus, but cannot be monitored in a SONET network, since the M13 apparatus and the ADM apparatus are different apparatuses.
That is, since the conventional M13 apparatus 2900 and the conventional ADM apparatus 2906 are configured independently, and both M13 apparatus 2900 and ADM apparatus 2906 are configured to transmit an alarm signal only to a DS3 network and a SONET network, respectively, the alarm information of the SONET network cannot be monitored from the DS3 network. The DS3 signal transmitted from the DS3 network is input to the DS3/DS2 demultiplexer unit 2902 of the M13 apparatus 2900, an alarm is detected by a DS3 alarm detector unit 2903, and the result of alarm detection is transmitted to the DSn condition reporting unit 2901. The DS2 signals output from the DS3/DS2 demultiplexer unit 2902 are input to a DS2/DS1 demultiplexer unit 2904, the signals are converted to DS1 signals, an alarm is detected by a DS2 alarm detector unit 2905, and the result of alarm detection is transmitted to a DSn condition reporting unit 2901. The DSn condition reporting unit 2901 is configured to transmit the result of the alarm detection to the DS3 network, and the detection result of the DS3 and DS2 alarms obtained in the M13 apparatus is not transmitted to the SONET network. To the conventional ADM apparatus 2906 are sent the DS1 signals demultiplexed and converted by the M13 apparatus 2900. In the ADM apparatus 2906 the DS1 signals are converted to VT1.5 signals by a DS1/VT1.5 multiplexer unit 2907, and a DS1 alarm detector unit 2908 detects the alarm of a DS1 signal and transmits the alarm to a SONET condition reporting unit 2910. The SONET condition reporting unit 2910 transmits the result of this alarm detection to the SONET network. On the other hand, the VT1.5 signals converted by the DS1/VT1.5 multiplexer unit 2907 are transmitted to a VT1.5/STS1 multiplexer unit 2909, and are converted to an STS1 signal, which is sent to the SONET network.
That is, the SONET network and the DSn network are formed independently of each other, and when a DSn network is connected to a SONET network using a conventional unit, only the information of the DSn signal at the contact point connected to the SONET network can be monitored.
Currently when a DS3 signal is connected to a SONET VT network, there is a demand for the condition monitoring and management of the DS2 and DS1 signals in the DS3 signal, even in the SONET VT network.
In one aspect of the present invention, the method for connecting to the SONET VT network by converting the DS3 (44,736 Mbps) signal to the DS1 (1,544 Mbps) signal once in the apparatus is utilized as means for connecting the DSn network to a SONET network. However, when converting the DS3 signal to the DS1 signal, the method described below is usually utilized in the conventional method. FIG. 4 shows the configuration of the system. The system comprises a DS3/DS2 demultiplexer unit 3000, a DS2/DS1 demultiplexer unit 3001 and a DS1/VT1.5 multiplexer unit 3002. The DS3 signal is terminated by a DS3 receiver unit 3003 of the DS3/DS2 demultiplexer unit 3000, and a DS3 alarm is detected. The alarm of the DS3 signal is inserted in a DS2 signal demultiplexed from the DS3 signal as an alarm indication signal by a DS2 transmitter unit 3005, and the DS2 signal is output. Furthermore, the DS2 signal is terminated by the DS2 receiver unit 3006 of the DS2/DS1 demultiplexer unit 3001, a DS2 alarm is detected, the alarm of the DS2 signal is inserted in the DS1 signal demultiplexed from the DS2 signal as an alarm indication signal of the DS1 signal by a DS1 transmitter unit 3008, and the DS1 signal is output. In the same way, in the DS1/VT1.5 multiplexer unit 3002 a transmitted DS1 signal is received by a DS1 receiver unit 3009, a DS1 alarm is detected, and the alarm of the DS1 signal is inserted in a VT1.5 signal multiplexed and converted from the DS1 signal as an alarm indication signal of the VT1.5 signal by a VT1.5 transmitter unit 3011, and the VT1.5 signal is transmitted.
When a DSn network is connected to a SONET network by combining a conventional M13 apparatus and a conventional ADM apparatus, this method is adopted. Currently, when this method is adopted, there are many processes of each of DS3, DS2 and DS1 signals, and the circuitry scale becomes large, since 7 channels of DS2 and 28 channels of DS1 of both alarm detector and alarm indication signal inserting circuits have to be provided.
In the same way, when in a conventional method a SONET network is connected to a DS3 (44.736 Mbps) network, a VT1.5 signal is extracted from an STS1 signal, and the VT1.5 signal is further converted to a DS1 signal, the following method is generally used. FIG. 5 shows the configuration of the system. The system comprises an STS1/VT1.5 demultiplexer unit 3100 and a VT1.5/DS1 demultiplexer unit 3101. An STS1 signal is terminated by the STS1 receiver unit 3103 of the STS1/VT1.5 demultiplexer unit 3100, an STS1 alarm is detected, the alarm of the STS1 signal is inserted in a VT1.5 signal extracted from the STS1 signal as the alarm indication signal of the VT1.5 signal by a VT1.5 transmitter unit 3105, and the VT1.5 signal is output. In the VT1.5/DS1 demultiplexer unit 3101 the signal is terminated by a VT1.5 receiver unit 3106, the alarm of the VT1.5 signal is detected, the VT1.5 signal is demultiplexed and converted to a DS1 signal, an alarm signal extracted from the VT1.5 signal is inserted in the DS1 signal as an alarm indication signal, and the DS1 signal is output. In a DS1/DS2 multiplexer unit 3102 the above-mentioned DS1 signal is received by a DS1 receiver unit 3109, an alarm is detected, the above-mentioned alarm is inserted in the DS2 signal as a DS1 alarm indication signal after the DS1 signal is multiplexed and converted to a DS2 signal, and the DS2 signal is transmitted from a DS2 transmitter unit 3111. In the same way, a converter unit for multiplexing and converting the DS2 signal to a DS3 signal is provided on the latter stage of the DS1/DS2 multiplexer unit 3102, and the DS3 signal is sent to the DS3 network.
When a conventional technology is used as a method to convert a DS3 (44.736 Mbps) to a DS1 (1.544 Mbps) once, and to then further convert to a VT1.5 signal, and to connect the signal to a SONET VT network, it is necessary to provide both a conventional ADM apparatus which can be connected to the SONET VT network using a DS1 signal, and a conventional M13 apparatus for carrying out DS3/DS1 conversion. In this case, the DS1 signal interfacing between a DS3/DS1 converter and a device for connecting a DS1 signal to a SONET VT network is an inter-apparatus communication. When forming the DS1 interface in order to make a DS1 inter-apparatus interface, as shown in FIG. 6, a DS1 digital data output unit 3200, a clock generator unit for DS1 output 3203, a DS1 uni-polar data output unit 3201, a DS1 uni-polar/bi-polar converter unit 3202, a DS1 bi-polar/uni-polar converter unit 3204, a DS1 uni-polar data input unit 3205 and a DS1 digital data input unit 3206 have to be provided, and in the M13 apparatus 3150 DS1 digital data extracted from a DSn network are B8ZS-encoded synchronized with a clock generated by the clock generator unit for DS1 output 3203 in the digital data output unit 3200, and are output to the DS1 uni-polar data output unit 3201. The encoded DS1 digital data are converted to DS1 uni-polar data by the DS1 uni-polar data output unit 3201, and are output to the DS2 uni-polar/bi-polar converter unit 3202. The DS1 uni-polar data are converted to DS1 bi-polar data by the DS1 uni-polar/bi-polar converter unit 3202, and are output to the DS1 bi-polar/uni-polar converter unit 3204. The DS1 bi-polar data are converted to DS1 uni-polar data by the DS1 bipolar/uni-polar converter unit 3204, and are output to the DS1 uni-polar data input unit 3205. In the DS1 uni-polar data input unit 3205 the DS1 uni-polar data are converted to DS1 digital data, and are output to the DS1 digital data input unit 3206. In the DS1 digital data input unit 3206 B8ZS-encoded data are decoded, and the decoded DS1 digital data are transmitted to the SONET network. The reverse process is executed in the same way. Currently, when this method is used, there are many signal processes performed on a DS1 signal, which leads to an increased number of signal processing circuits.
FIG. 7 shows the case where in a prior art STS1, VT1.5 and DS1 signals from a SONET VT network are converted in that order, DS1, DS2 and DS3 signals are multiplexed in that order, and the DS3 signal is output to a DSn network. The system comprises an STS1/VT1.5 demultiplexer unit 3300, a VT1.5/DS1 demultiplexer unit 3301, a clock generator unit for DS3 network 3303, a DS1/DS2 multiplexer unit 3302 and a DS2/DS3 multiplexer unit 3304. In the STS1/VT1.5 demultiplexer unit 3300 an input STS1 signal is terminated. The terminated STS1 signal is demultiplexed and converted to VT1.5 signals, which are output to the VT1.5/DS1 demultiplexer unit 3301. The terminated VT1.5 signals are demultiplexed and converted to DS1 signals, which are output to the DS1/DS2 multiplexer unit 3302. In the DS1/DS2 multiplexer 3302 the DS1 signals are multiplexed and converted to DS2 signals synchronized with a clock generated by the clock generator unit for DS3 network 3303, which are output to the DS2/DS3 multiplexer unit 3304. Although an STS1 signal in the SONET network generated is at this moment demultiplexed and converted, and the frequency offset generated by the fluctuation in the bit number at a destuff process at the time of destuf f-demultiplexing is stored in the DS1, the frequency offset is absorbed by a stuffing process when stuff-multiplexing and converted to a DSn signal. There is also a method where in the DS2/DS3 multiplexer unit 3304 the input DS2 signals are multiplexed and converted to a DS3 signal, which is output to a DS3 network. In this case, 8 bits of bit stuff to be used for the stuffing process of a SONET signal cause an instantaneous frequency offset in the DS1 signal at the time of destuffing, which in turn causes jitters in the DS1 signal. When the DS1 signals are multiplexed and converted to DS2 signals, and then to a DS3 signal in order, these jitters are absorbed by 1 bit of bit stuff to be used in a stuffing process of a DSn signal. However, when 8 bits of stuff bit are frequently generated by the destuffing process of a SONET signal, the jitters caused by the frequency offset generated by the destuffing process cannot be absorbed by 1 bit of stuff bit of a DSn signal, and a signal error may be caused.